A storage medium stores data for use by an electronic device having a reading device. Common examples of storage media include floppy disks, optical disks, hard disks, and the like. Common examples of a reading device are a floppy disk drive, a hard disk drive, and the like. A typical storage medium may be formatted to contain thousands of “tracks” of information, organized as concentric rings on the storage medium surface. These tracks must be precisely followed by the electronics of the reading device during reading and writing of data to/from the storage medium surface. If the tracks are not precisely followed with appropriate position and clocking speed, data may be erroneously read. Information is typically encoded on the storage medium to facilitate proper tracking. Various information formats have been employed; however, as storage density increases, existing formats become less practical.
As background on these formats, in a typical track following technique, for an optical storage medium, the medium contains data tracks as well as land and grooves marks which provides continuous tracking information to the electronics of the reading device. A first sensor reads the data tracks and a second sensor reads the land and groove marks. The land and groove marks are used by the electronic device to maintain the proper clocking speed and position of the reading device, thereby allowing the device to accurately read data from the storage medium (i.e., with minimal read and/or write errors).
In a typical track following technique, for a magnetic storage medium, the electronics of the reading device follows the tracks via servo sectors in a single track, embedded at intervals in the data, around the track (typically referred to as embedded servo). As such, the storage medium includes both data sectors and servo sectors, both of which may be read by a single read head. The data sector is used to store data marks and the servo sector is used to store servo marks. Servo marks are used by the electronic device to maintain the proper circumferential timing information and position of the reading device, thereby allowing the device to accurately read data from the storage medium (i.e., with minimal read and/or write errors).
Applying the embedded servo technique that is typically used for magnetic storage medium to an optical storage medium (which typically has a higher storage density), provides several design challenges. This is especially desired in a near field optical storage medium where there is no way to discriminate data patterns from the diffraction patterns from a land or groove that is commonly used in an optical storage medium. First, a straightforward application of known servo marks to an optical storage medium creates a large amount of servo overhead. That is, the ratio of servo mark area to data mark area may become unacceptable, as described in more detail below.
As background on the servo marks, in a typical magnetic storage medium, servo marks occur 60 to 150 times per revolution taking 4 to 12% of the total disk storage area. In a high track optical drive, which has a 1 to 3 KHz bandwidth servo, a sample rate of 10 to 30 KHz is used, which in turn uses 200 to 1000 servo samples per revolution. In such a system, servo marks would occupy 12 to 50% of the total disk storage space. This is an undesirable and sometimes unacceptable amount of servo overhead.
Further, in magnetic storage media today, a new data synchronization field occurs after every servo mark and increases servo mark overhead. If more servo marks are added to increase the servo bandwidth the re-synchronization field overhead is even worse.
In optical storage, wobble marks may be used by a reading device to remain aligned with a data track of a storage medium. Wobble marks are typically located offset from the center line of a data track. Typically, one wobble mark is located on one side of the center line of a data track and another wobble mark is located on the other side of the center line of the data track. The reading device receives a signal by reading the wobble marks and adjusts the position of the reading device according to the signal. If the reading device becomes mis-aligned with the data track, then the reading device may begin to incorrectly read the data marks. Additional wobble marks may be located on the track center line of a data track to provide additional information to the reading device. Conventional magnetic servo mark formats include both types of wobble/servo marks in one servo sector. However, including both types of wobble marks in one servo sector increases the size of the servo sector. Moreover, some conventional magnetic servo mark formats include multiple wobble marks in one servo sector, which are averaged. This technique, while providing some benefit, may create an unacceptable amount of servo overhead in a high density storage medium.
Greycodes are used by a reading device to determine a track number in a magnetic storage medium. A servo mark on each track may include a plurality of bits representing the track number of that particular track. In that manner, the reading device can determine its radial position relative to the storage medium without having to return to a known position (e.g., the zero track). This can reduce recovery time if the reading device loses track of its radial position. Conventional greycodes have a specific field within the servo sector dedicated to the greycode. However, this dedicated field increases the size of the servo sector and if applied to a high sample rate, high density storage device, may increase servo overhead to an unacceptable level.
Reset/servo sync marks indicate the location of a servo sector and are used to distinguish the servo sector from the data sector. The reset mark is typically followed by a gap having no marks so that the reading device can confirm that the reading device head is located in a servo sector.
As can be seen, a storage medium typically includes data sectors and servo sectors. Data sectors are used to store data and servo sectors are important in accurately reading the data from the storage medium, however, there is a tradeoff between the amount of area of a storage medium used for data sectors and the amount of area used for servo sectors. It is desired to have the maximum amount of data on a storage medium; however, without enough servo sectors, the data sectors may not be read accurately. As such, servo marks can be viewed as storage media overhead. To maximize the data of a storage medium, the amount of servo mark area should be minimized, especially when applied to a high sample rate, high density storage medium. However, if too few servo marks are used, a reading device may not be able to accurately read the data marks from the storage medium.
As can be seen, as the density of information stored on a storage medium becomes greater, the issue of servo mark overhead becomes critical. Each servo mark decreases the amount of storage space available for data marks on the storage medium and a relatively long servo mark decreases the amount of data storage space even more. Long servo marks may cause a reading device to become unsynchronized within a servo sector and if this occurs, the reading device requires a predefined data resynchronization pattern that increases the overhead of retrieving data. Applying a conventional servo mark format to a high sample rate, high density storage medium may result in excessive servo mark overhead, thereby reducing the amount of storage available for data marks.
Further, to distinguish servo marks from data marks, servo marks are typically placed on the storage medium at one frequency and data marks are typically placed on the storage medium at another frequency. For example, data marks may be placed on the storage medium every five clock cycles and servo marks may be placed on the storage medium every fifteen clock cycles. This difference in spacing may be used by the reading device to differentiate between servo marks and data marks. A data clock cycle is used to read data marks and a servo clock cycle is used to read servo marks. If the ratio between the data clock and the servo clock are known, then the servo clock can be multiplied by a constant to derive the data clock. This provides a data clock, servo clock pair that are tightly coupled and therefore, should minimize read and write errors. However, when banded storage (i.e., the concentric tracks are divided into bands or zones) is implemented on a storage medium, data mark spacing varies with the particular zone. Therefore, multiplying the data clock by a constant does not provide a servo clock that functions for all zones.
In view of the above problems, there is a recognized need for a system and method for a storage format for a high bandwidth servo system. The present invention satisfies this need.